1. Field of the Invention
In one aspect, the invention relates to compounds that are inhibitors of Raf kinase, as well as compositions containing these compounds and methods of use. The compounds are useful for inhibiting Raf kinase and for treating disorders mediated thereby. The invention also relates to methods of using the compounds of the present invention for in vitro, in situ, and in vivo diagnosis or treatment of mammalian cells, or associated pathological conditions.
2. Description of the State of the Art
The Raf/MEK/ERK (extracellular signal-regulated kinase) kinase cascade is pivotal in transmitting signals from membrane receptors to transcription factors that control gene expression culminating in the regulation of cell cycle progression (Robinson, M J and Cobb, M H (1997) Curr. Opin. Cell Biol. 9:180-186). This cascade can prevent cell death through ERK2 and p90(Rsk) activation and phosphorylation of apoptotic and cell cycle regulatory proteins (Shelton, J G et al. (2003) Oncogene 22(16):2478-92). The PI3K/Akt kinase cascade also controls apoptosis and can phosphorylate many apoptotic and cell cycle regulatory proteins. These pathways are interwoven as Akt can phosphorylate Raf and result in its inactivation, and Raf can be required for the anti-apoptotic effects of Akt. Raf is a key serine-threonine protein kinase which participates in the transmission of growth, anti-apoptotic and differentiation messages. These signals can be initiated after receptor ligation and are transmitted to members of the MAP kinase cascade that subsequently activate transcription factors controlling gene expression. Raf is a multigene family which expresses oncoprotein kinases: Raf-1, A-Raf and B-Raf (McCubrey, J A., et al. (1998) Leukemia 12(12):1903-1929; Ikawa, et al. (1988) Mol. and Cell. Biol. 8(6):2651-2654; Sithanandam, et al. (1990) Oncogene 5:1775-1780; Konishi, et al. (1995) Biochem. and Biophys. Res. Comm. 216(2):526-534). All three Raf kinases are functionally present in certain human hematopoietic cells, and their aberrant expression can result in abrogation of cytokine dependency. Their regulatory mechanisms differ because C-Raf and A-Raf require additional serine and tyrosine phosphorylation within the N region of the kinase domain for full activity (Mason et al. (1999) EMBO J. 18:2137-2148), and B-Raf has a much higher basal kinase activity than either A-Raf or C-Raf. The three Raf oncoproteins play critical roles in the transmission of mitogenic and anti-apoptotic signals. Recently, it has been shown that B-Raf is frequently mutated in various human cancers (Wan, et al. (2004) Cell 116:855-867). Development of specific Raf inhibitors may prove efficacious in cancer therapy. The cytoplasmic serine/threonine kinase B-Raf and receptor tyrosine kinases of the platelet-derived growth factor receptor (PDGFR) family are frequently activated in cancer by mutations of an equivalent amino acid. Structural studies have provided important insights into why these very different kinases share similar oncogenic hot spots and why the PDGFR juxtamembrane region is also a frequent oncogenic target (Dibb, NJ (2004) Nature Reviews Cancer 4(9):718-27).
Transformation of normal melanocytes into melanoma cells is accomplished by the activation of growth stimulatory pathways, typically leading to cellular proliferation, and the inactivation of apoptotic and tumor suppressor pathways. Small molecule inhibitors of proteins in the growth stimulatory pathways are under active investigation, and their application to melanoma patients would represent a new treatment strategy to inhibit cell proliferation or induce cell death (Polsky, D., (2003) Oncogene 22(20):3087-3091; Konopleva, M., et al. (2003) Blood 102(11):625a).
B-Raf encodes a RAS-regulated kinase that mediates cell growth and malignant transformation kinase pathway activation. Activating B-Raf mutations have been identified in 66% of melanomas and a smaller percentage of many other human cancers. B-Raf mutations also account for the MAP kinase pathway activation common in non-small cell lung carcinomas (NSCLCs), including V600E and other mutations identified as novel, altering residues important in AKT-mediated B-Raf phosphorylation, which suggest that disruption of AKT-induced B-Raf inhibition can play a role in malignant transformation. Although >90% of B-Raf mutations in melanoma involve codon 600 (57 of 60), 8 of 9 B-Raf mutations reported to date in NSCLC are non-V600 (89%; P<10(−7)), strongly suggesting that B-Raf mutations in NSCLC are qualitatively different from those in melanoma; thus, there may be therapeutic differences between lung cancer and melanoma in response to RAF inhibitors. Although uncommon, B-Raf mutations in human lung cancers may identify a subset of tumors sensitive to targeted therapy (Brose, M S, et al., (2002) Cancer Research 62(23):6997-7000).
Raf protein kinases are key components of signal transduction pathways by which specific extracellular stimuli elicit precise cellular responses in mammalian cells. Activated cell surface receptors activate ras/rap proteins at the inner aspect of the plasma membrane, which in turn recruit and activate Raf proteins. Activated Raf proteins phosphorylate and activate the intracellular protein kinases MEK1 and MEK2. In turn, activated MEKs catalyze phosphorylation and activation of p42/p44 mitogen-activated protein kinase (MAPK). A variety of cytoplasmic and nuclear substrates of activated MAPK are known which directly or indirectly contribute to the cellular response to environmental change. Three distinct genes have been identified in mammals that encode Raf proteins; A-Raf, B-Raf and C-Raf (also known as Raf-1) and isoformic variants that result from differential splicing of mRNA are known.
Inhibitors of Raf kinases have been suggested for use in disruption of tumor cell growth and hence in the treatment of cancers, e.g., histiocytic lymphoma, lung adenocarcinoma, small cell lung cancer and pancreatic and breast carcinoma; and also in the treatment and/or prophylaxis of disorders associated with neuronal degeneration resulting from ischemic events, including cerebral ischemia after cardiac arrest, stroke and multi-infarct dementia and also after cerebral ischemic events such as those resulting from head injury, surgery and/or during childbirth (neurotrauma). In particular, it has been suggested that B-Raf is the major Raf isoform activated by the neurotrophin, nerve growth factor (NGF), for NGF induced extracellular signaling by kinase activation (York, et al. (2000) Mol. and Cell. Biol. 20(21 ):8069-8083).